An MCCB is an electrical protection device that can be considered the "big brother" of a miniature circuit breaker (MCB)—miniature circuit breakers are commonly used in small devices in homes or offices. They are typically used when the load current exceeds the capacity of the MCB, or in applications where the tripping value needs to be adjustable.
While some versions offer tripping ratings of only tens of amperes, MCCBs are generally much larger than MCBs and can handle currents up to thousands of amperes. Like all types of thermal-magnetic circuit breakers, MCCBs have two main protection modes:
Overload protection is achieved using a bimetallic strip. Under normal operating conditions, current flows continuously through this strip. However, if a large current flows for an extended period, the heating effect causes the strip to bend, breaking the contacts and cutting off power to the load.
- Fault protection is an electromagnetic function with instantaneous response, enabling immediate interruption upon encountering a fault current. Whenever a fault occurs, the electromagnetic coil within the MCCB is energized, causing the contacts to trip and disconnecting the power supply to the circuit. Due to the large current to be interrupted, many MCCBs include arc suppression measures.
The MCCB also includes a circuit breaker that can be manually tripped. This switch is used to disconnect the equipment from the power supply during maintenance or system upgrades.
Molded case circuit breakers require reliable switching technology.
Recent research and development on MCCBs has focused on enhancing connectivity, enabling remote control and monitoring. Many MCCB manufacturers now offer smart modules compliant with Plant 4.0 standards, allowing for remote monitoring and metering of parameters such as phase current and voltage. This also helps in calculating electricity and energy usage, giving plant operators greater insight into their operating costs.
MCCB switch
The switches in an MCCB serve two main functions: they are typically used to set key parameters such as trip current and delay, and they act as auxiliary switches to perform various functions once the MCCB trips. This requires the use of two very different types of switches.
- To set the trip current, a PCB-mounted dual in-line package (DIP) switch is typically used. These switches can be rotary (most common) or, occasionally, standard through-hole piano key DIP switches. These switches are rarely used, so reliability is not critical here. However, ensuring the correct current setting is essential for safety, preventing nuisance trips, and ensuring that no downstream equipment is damaged in the event of a failure.
To achieve this, rotary switches are designed with contacts that have a positive brake, ensuring the actuator can only be set at integer points on the DIP switch. Generally, setting these switches requires a tool such as a small screwdriver. Because the switches are very small and can be difficult to reach, using a tool to turn the actuator is easier than operating it with your fingers. It also provides a degree of tamper protection. Many manufacturers offer various types of actuators as semi-custom options, allowing MCCB manufacturers to choose the execution method or whether special tools are required.
- Auxiliary switches are typically microswitches used for notification in the event of a trip and for controlling external devices. While their usage may be lower compared to other applications, controlling a variety of devices can be challenging for designers. Controlling alarm lights, relays, and external power supplies may require currents in the range of 10A to 15A, while controlling PLCs and circuits may require lower signal currents. Incorporating this dual-function design into a general-purpose device (for configuration flexibility) often necessitates custom designs rather than standard switches.
Molded case circuit breakers require reliable switching technology.
A key requirement for auxiliary switches is mechanical compatibility with MCCBs so that they can be easily and quickly removed if the end customer wishes to reconfigure various functions. This typically requires customizing standard microswitches to add an overmolded housing with a quick-release latch. Custom wiring is also often added to suit specific applications and facilitate installation.
Popular switches in MCCB applications
The two main types of switches used in MCCB are:
RTE Series – Ultra-thin Rotary DIP Switches
This flexible, multi-functional rotary DIP switch is primarily used in MCCBs to set basic parameters such as trip current.
The RTE can be selected for 4, 10, or 16 positions, and the output can be specified as BCD, Gray code, hexadecimal code, or a simple single-pole output, depending on the application requirements. This switch is available in a range of mechanical arrangements, including vertical, horizontal, and reverse versions, and a surface-mount horizontal version is also available.
The standard LCP screwdriver actuator can be used with either straight or Phillips head screwdrivers and features a replaceable handle for manual operation in applications where switches are easily accessible. The top plate is available in silver or black for improved aesthetics and legibility, and an ESD protection plate is also available as an option.
The moving contact has very low resistance, only about 100mΩ, and is made of gold-plated copper alloy to ensure reliability. Its shape provides a positive brake for each position, thus ensuring reliable setting to the correct position. The RTE switch has a mechanical life of approximately 20,000 indexations and can operate in ambient temperatures ranging from -25°C to +85°C.
TM Series – Miniature Quick-Act Switches
For switches used in MCCBs to indicate that the MCCB has tripped, the UL-rated™ series forms the basis for many custom snap-on solutions.
The single-pole instantaneous switch is rated for 15A/250VAC and is available in SPST (normally open and normally closed) and SPDT (single-pole double-throw) configurations. For applications requiring only signal-level switching, a low-current version with gold-plated contacts is also available (pure silver plating for current ratings greater than 1A).
Designers can specify the operating force (three options), the center position of the actuator, and the type of actuator (lever, roller, needle roller, or analog roller) to meet the mechanical requirements of almost any application.
These switches boast a cycle life of up to 350,000 cycles and an operating temperature range of -55°C to +150°C, making them ideal for harsh industrial applications. They are available with a range of terminals, including two sizes of quick-connect solder terminals and screw terminals—allowing for lead termination to meet the requirements of factory-produced custom components.
Summarize
MCCBs are becoming increasingly intelligent, enabling them to provide monitoring and management capabilities in Factory 4.0 scenarios. However, these thermoelectric devices still heavily rely on mechanical switches to perform critical functions, such as setting key parameters and providing trip notifications.
